For details about this issue see issue #151498.
java.lang.IllegalStateException: Should not acquire Children.MUTEX while holding ProjectManager.mutex()
at org.openide.nodes.Children$ProjectManagerDeadlockDetector.execute(Children.java:1808)
at org.openide.util.Mutex.postRequest(Mutex.java:1245)
at org.openide.util.Mutex.postRequest(Mutex.java:1209)
at org.openide.util.Mutex.postWriteRequest(Mutex.java:582)
at org.openide.nodes.Children$Keys.applyKeys(Children.java:1513)
at org.openide.nodes.Children$Keys.setKeys(Children.java:1457)
at org.netbeans.spi.java.project.support.ui.PackageViewChildren.refreshKeys(PackageViewChildren.java:244)
at org.netbeans.spi.java.project.support.ui.PackageViewChildren.fileFolderCreated(PackageViewChildren.java:396)
Caused: java.lang.reflect.InvocationTargetException
at sun.reflect.GeneratedMethodAccessor60.invoke(GeneratedMethodAccessor60.java:0)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.openide.util.WeakListenerImpl$ProxyListener.invoke(WeakListenerImpl.java:451)
Caused: java.lang.reflect.UndeclaredThrowableException
at $Proxy1.fileFolderCreated(.java:0)
at org.openide.filesystems.FCLSupport$DispatchEventWrapper.dispatchEventImpl(FCLSupport.java:133)
at org.openide.filesystems.FCLSupport$DispatchEventWrapper.dispatchEvent(FCLSupport.java:122)
at org.openide.filesystems.FCLSupport.dispatchEvent(FCLSupport.java:99)
at org.openide.filesystems.FileObject$ED.dispatch(FileObject.java:1052)
at org.openide.filesystems.EventControl.invokeDispatchers(EventControl.java:203)
at org.openide.filesystems.EventControl.exitAtomicAction(EventControl.java:177)
at org.openide.filesystems.EventControl.runAtomicAction(EventControl.java:122)
at org.openide.filesystems.FileSystem.runAtomicAction(FileSystem.java:509)
at org.netbeans.spi.project.support.ant.AntProjectHelper.runSaveAA(AntProjectHelper.java:318)
at org.netbeans.spi.project.support.ant.AntProjectHelper.saveXml(AntProjectHelper.java:334)
at org.netbeans.spi.project.support.ant.AntProjectHelper.save(AntProjectHelper.java:606)
at org.netbeans.spi.project.support.ant.AntProjectHelper.access$100(AntProjectHelper.java:103)
at org.netbeans.spi.project.support.ant.AntProjectHelper$1.save(AntProjectHelper.java:141)
at
org.netbeans.modules.project.ant.AntBasedProjectFactorySingleton.saveProject(AntBasedProjectFactorySingleton.java:360)
at org.netbeans.api.project.ProjectManager$6.run(ProjectManager.java:643)
at org.netbeans.api.project.ProjectManager$6.run(ProjectManager.java:632)
at org.openide.util.Mutex.writeAccess(Mutex.java:433)
at org.netbeans.api.project.ProjectManager.saveProject(ProjectManager.java:632)
at org.netbeans.modules.project.ui.OpenProjectList.close(OpenProjectList.java:793)
at org.netbeans.modules.project.ui.actions.CloseProject.actionPerformed(CloseProject.java:73)
at org.netbeans.modules.project.ui.actions.LookupSensitiveAction.actionPerformed(LookupSensitiveAction.java:166)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
...